Over 1,000 migrant workers were arrested after Thai police raided factories suspected of employing them.
Immigrants are required to register to work in the country.
[Sanan Kachornprasat, Deputy Thai Prime Minister]:
"Thailand needs to be strict with this matter otherwise the number of crimes will increase. We already announced to migrant workers who have been employed by companies to register so they will be legal migrant workers."
Most of the illegal migrant workers are from Burma, Laos and Cambodia.
Police say all detained migrant workers will be investigated.
There has been an increase in raids since the government pressed forward to implement a new migrant labor policy.
Migrant workers are a key source of manual labor in Thailand, especially in the construction, tourism and manufacturing sectors.